Magnesia is an important compound that finds application in various industries. Magnesium oxide has the highest melting point of the moderately priced oxides and is therefore an important raw material for refractory bricks and other materials. It is the only material apart from ZrO2 that can withstand long-term hearing above 2000° C.
Reference may be made to Ullmann's Encyclopedia, 6th Edition (electronic version) wherein it is stated that: “The increased demands made on refractory materials as a result of higher operating temperatures and shorter tap to tap times in metallurgical furnaces and reactors can only be met by pure, high-density magnesia sinters.” Small quantities of “contaminants” are disadvantageous if they form low-melting eutectics with MgO (e.g., with CMS at 1485° C. or with C2F at 1200° C. because this leads to deterioration of mechanical properties (e.g., strength and volume stability) at high temperatures. High-quality sinters therefore have a low content of high-melting silicate phases (such as C2S), a low B2O3 content, and a high degree of direct periclase-periclase contact (without intermediate silicate phases).
Magnesia bricks have a high heat storage capacity and a high thermal conductivity. They are used in efficient off-peak storage heaters. The heat generated by a heating element is transferred to the magnesia brick and increases its temperature. Thermal conductivity is increased by a high periclase content and a low porosity. The specific heat is only slightly lowered by SiO2 and Al2O3, but is significantly lowered by CaO, Cr2O3, and Fe2O3. The bricks should not contain free CaO (risk of hydration) or crystal phases with different modifications.
Caustic magnesia was formerly produced exclusively from cryptocrystalline magnesite with a low iron content but is now also obtained from all types of magnesite and Mg(OH)2. Its MgO content ranges from ca. 65 to 99 wt %, and may even reach 99.9%. The magnesia is often ground prior to use. Extremely reactive caustic magnesia may have a surface area of up to 160 m2/g. Depending on the burning temperature, the product is termed light burned (870-1000° C.) or hard burned (1550-1650° C.). Light-burned, caustic magnesia becomes hydrated in cold water and is soluble in dilute acid. It has a loose bulk density of 0.3-0.5 g/cm3 and a specific surface area (BET) of 10-65 m2/g. Hardburned caustic magnesia has a loose bulk density of 1.2 g/cm3 (bulk density 2 g/cm3).
MgO can be pressure hydrated to form Mg(OH)2. It can also be converted into anhydrous MgCl2 through the reaction of eq. 1 (Electrolytic Production of Magnesium, Kh. L. Strelets, Keter Publishing House Jerusalem Ltd., 1977, p28)MgO+Cl2+CO→MgCl2+CO2+70.8 cal/mole  (eq. 1)and the anhydrous MgCl2 can be converted into Mg and Cl2 by electrolysis (eq. 2)MgCl2→Mg+Cl2  (eq. 2).Alternatively, MgO can be thermally reduced to obtain Mg.
Reference is made to Ullmann's Encyclopedia wherein it is reported that magnesia can be prepared by the decomposition of magnesite (MgCO3). The main drawback of this method is that magnesite ore can have high levels of impurity. The highest quality magnesites, particularly those for refractory applications, are needed for a magnesia product with a high MgO content, a CaO:SiO2 mass ratio of 2-3, and low contents of Fe2O3 and Al2O3. The presence of accompanying, low-melting minerals can adversely affect the properties of the sintered magnesia.
Reference may also be made to a publication entitled “Magnesite-A market survey” published by Indian Bureau of Mines, Nagpur and “Magnesite” in Indian Minerals Year Book, Vol.-2 (1989) published by Indian Bureau of Mines, Nagpur, pages 698 to 699, wherein magnesia is prepared by calcination of naturally occurring magnesite deposits. The drawback of this process is that magnesite ores contain varying amount of silica, iron oxide, alumina, and lime as silicates, carbonates, and oxides. Selectively mined ore is passed through various beneficiation methods like crushing and size separation, heavy media separation, and froth flotation to reduce lime and silica content prior to calcination. Magnetic separation reduces iron concentration but is effective only when the iron is present in the form of discrete ferromagnetic minerals rather than as ferrous carbonate. Due to all this, high purity magnesia is difficult to produce by this process.
Reference is made to the Sulmag II process (W. S. Ainscow: “Aufbereitung von Magnesit zu hochwertiger Sintermagnesia,” TIZ 110 (1986) no. 6, 363-368. Sulmag II the Sinter Magnesite Process, Sulzer Brothers Ltd., Winterthur, Switzerland) for producing light-burned caustic magnesia in a gas suspension kiln from low-magnesite ores. Dissolved magnesium chloride is obtained by selective extraction with recycled NH4Cl solution (eqs. 3, 4) and all insoluble impurities are removed through filtration. Needle-shaped crystals of nesquehonite (MgCO3.3H2O) are precipitated out in the reactor and filtered (eq. 5). Caustic magnesia with a high specific surface area is obtained by heating the nesquehonite.MgCO3→MgO+CO2  (eq. 3)2NH4Cl+MgO+H2O+Contaminants→2NH4OH+MgCl2+Tailings  (eq. 4)MgCl2+(NH4)2CO3+3H2O→MgCO3.3H2O↓+2NH4Cl  (eq. 5)
The above process has many advantages but would give product of very low bulk density which may not be suitable in refractory applications, which comprises the bulk of applications related to magnesia.
Reference may also be made to the technique of pyrohydrolysis. MgCl2-rich brine is purified to remove bromide and traces of boron and then fed via steel pipes into the spray nozzles of the reactor. It is sprayed into the cylindrical, externally insulated reactor at ca. 600° C. The water evaporates from the atomized brine droplets leaving a perforated chloride crust which reacts with the steam to form MgO and HCl. The crude product is washed with water and hydrated in a stirred tank, and then concentrated in a thickener. The resulting slurry is difficult to filter and is washed and dewatered in a two-stage vacuum drum filter. The calcined product typically contains ≧99.5 wt % MgO, <1 wt % CaO, ≦0.05 wt % SiO2, ≦0.05 wt % Fe2O3, ≦0.005 wt % Al2O3, and ≦0.01% B2O3; its specific surface area is 2-50 m2/g, the loose bulk density ranges from 0.8 to 0.2 g/cm3. The main drawback is that spray calcination is an energy intensive process and choking up of nozzles can pose a problem. Another drawback is that the MgO obtained after first calcination leads to a slurry that is indicated to be “difficult to filter” which would largely offset any advantage that might be gained.
Reference may be made to the U.S. Pat. No. 4,255,399 dated Mar. 10, 1981 entitled “Process for the Recovery of Magnesium Oxide of High Purity” by Grill et. al, wherein magnesium oxide is obtained by thermal decomposition of a magnesium chloride brine previously purified. Concentrated magnesium chloride is decomposed in a thermal reactor where hot gases convert it into magnesium oxide and hydrochloric acid. The problems no doubt would be similar to those stated above.
Reference is made to U.S. Pat. No. 6,776,972, dated Aug. 17, 2004, wherein Vohra et al. have described the use of HCl gas generated from spray pyrolysis for reaction with limestone to prepare CaCl2 which can then be used to desulfate sea/sub-soil bittern for the facile production of carnallite double salt wherefrom KCl can be produced. The problem of spray calcination, however, remains.
Reference may be made to “Preparation of magnesium hydroxide flame retardant by ammonia method,” by Li, Kemin; Zhang, Li; Wujiyan Gongye, (33(2), 14-16 (Chinese) 2001 Wujiyan Gongye Bianjib, CA 135:115882; CA Section: 78 (Inorganic Chemicals and Reactions), wherein the flame retardant was prepared by allowing bittern after recycling K2SO4 to react with NH4OH, hydrothermal treatment to obtain Mg(OH)2, treating by surface treatment, washing, drying, and crushing. The content of Mg(OH)2 of the flame retardant was 97%. No mention is made of any process that produces MgO from the crude unwashed Mg(OH)2.
Reference may be made to “Recovery of magnesium hydroxide, gypsum and other products from natural and technical brines, in particular from final lyes of potash works” by Krupp, Ralf (Germany) (Ger. Offen. DE 10154004 Al 15 May 2003, 9 pp. (German); CA 138:371080), wherein, recovery of Mg(OH)2 and gypsum from MgSO4-and MgCl2-containing brines results by precipitation of Mg-ions with NH3 or NH4OH. Gaseous NH3 is recovered by addition of CaO and recycled for the precipitation step. The method allows the manufacture of Mg(OH)2 without impurities such as Fe, Mn, Al, and Ca. However, although not stated explicitly, the preparation of pure Mg(OH)2 would no doubt have involved the washing of the solid to remove adhering NH4Cl, MgCl2, etc.
Reference may be made to “One-step process for manufacture of magnesium hydroxide” by Wang, Fuwen; Zhang, Jun; Liu, Jianhua; Dong, Yijun (Shandong Haihua Group Corp., Ltd., Peop. Rep. China). Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1332117 A 23 Jan. 2002, 7 pp. (Chinese) (People's Republic of China). Bittern containing MgCl2 and ammonium hydroxide[mol ratio of MgCl2/ammonia=1/(1.3-2.0)] are reacted at 45-90° for 5-30 minutes, filtered, washed, dried, and pulverized to give solid magnesium hydroxide. No mention is made of the difficulties encountered in purifying Mg(OH)2 besides the disadvantage of using ammonia vis-á-vis inexpensive lime. Seawater contains magnesium and has the inherent advantage of having virtually no silica contamination. Thus high quality Mg(OH)2 can be produced mainly using seawater/brine/bittern of marine origin.
Reference may be made to the paper by J. A. Fernandez-Lozano entitled “Utilization of Seawater Brines for the Production of High Purity Magnesium Oxide and Magnesium Hydroxide” published in the Proceedings of the Fifth International Symposium on Salt—Northern Ohio Geological Society, 1979, pp 269-279 wherein the author has stated that Mg(OH)2 obtainable from the reaction of MgCl2-rich seawater brine and ammonia can be made of high purity by washing the hydroxide and that, in principle, MgO of high purity can be obtained as a result. No mention is made of the difficulties encountered in purifying Mg(OH)2 besides the disadvantage of using ammonia vis-á-vis inexpensive lime.
Reference is made to the preparation of MgO from Mg(OH)2 by calcination (eq. 6).Mg(OH)2→MgO+H2O  (eq. 6)
Reference is also made to Kirk Othmer, Encyclopedia of Chemical Technology, 4th Ed., Vol. 15, p 690 wherein it is stated that “To precipitate and recover magnesium hydroxide from solutions of magnesium salts, a strong base is added. The more commonly used base is calcium hydroxide derived from lime (CaO) or dolime (CaO—MgO).” Sodium hydroxide is used as a precipitant if a product having low CaO content is desired.
Reference may be made to the paper entitled “Carbonation of Aqueous Suspensions containing Magnesium Oxides or Hydroxides” by Robert L. Evans and Hillary W. St. Clair in “Industrial and Engineering Chemistry” 1949, 41(12), 2814-2817, wherein a modification of the Pattinson process (carbonation of magnesium hydroxide to magnesium bicarbonate) is described. A suspension of magnesium hydroxide is carbonated to form a metastable solution of magnesium bicarbonate. After the separation of insoluble impurities, the solution is decarbonated by heating or aeration and the magnesium carbonate precipitates as trihydrate, the penta hydrate or the basic carbonate. The precipitate is recovered from the solution by filtration and converted to magnesium oxide by thermal decomposition. The main drawback of the process is that the process is very sensitive to the partial pressure of carbon dioxide and to the temperature. The stability of the metastable solution of magnesium bicarbonate decreases markedly as the temperature rises above normal room temperature. Moreover, the bulk density of the MgO would be too low for refractory applications.
Reference may be made to the paper “Chemical Engineering Problems in the Sea Water Magnesia Process” read by H. W. Thorp and W. C. Gilpin at a meeting of the Chemical Engineering Group, held in the Apartment of the Geological Society, Burlington House, London, W. I. on Tuesday, Oct. 25, 1949 wherein the recovery of magnesia from sea water lies in the difficulty of precipitating the magnesium hydroxide in a form which will settle rapidly and which will yield a sludge easy to de-water. It is realized that each ton of magnesia must be separated from some 300 tons of water, which amount does not include any used for washing the precipitate. It is necessary to ensure the minimum contamination by lime; the sea water is treated prior to the removal of the magnesium hydroxide, with a small proportion of lime to precipitate the bicarbonate ion as calcium carbonate.
Reference is made to Ullmann's Encyclopedia wherein the production of MgO from seawater and brines is described such that 470 m3 of seawater are required to produce 1 t of MgO; and in practice 600 m3 are needed. The process is based on the precipitation of magnesium hydroxide (solubility in water 0.0009 wt %) by addition of calcium hydroxide (solubility 0.185 wt %):Mg2++2Cl−+Ca(OH)2→Mg(OH)2↓+Ca2++2Cl−
The main drawbacks of the process are that a supply of freshwater (>40 m3 per tonne MgO) is required to wash the Mg(OH)2 and to produce the milk of lime. High-purity limestone or dolomite deposits should be available in the vicinity; they are calcined and slaked to provide Ca(OH)2 as the precipitating agent and should therefore contain minimal quantities of elements that form insoluble carbonates, sulfates, etc. The freshwater also requires to be decarbonated. Unless specially treated, caustic and sintered magnesia produced from seawater usually contain ca. 0.2% B2O3 and small amounts of CaO, SiO2, Al2O3, and Fe2O3 derived from the limestone or wastes in the seawater. The B2O3 content of the magnesia is also generally lowered to ca. 0.05% by using a 5-12% excess of lime for precipitation (overliming); this increases the pH to 12 and minimizes the adsorption of boron. Apart from all the obvious drawbacks no mention is made of the difficulties of washing Mg(OH)2 which problem is even more complex as a result of overliming.
Reference may also be made to the paper “Recovery of Magnesium Compounds from Sea Water” by W. C. Gilpin and N. Heasman in “Chemistry and Industry,” 6 Jul. 1977, 567-572, wherein the process of recovering magnesia from seawater and the problems with the process are clearly outlined. The drawbacks of the process are similar to those described above.
It will be observed from the prior art that wherever Mg(OH)2 has been used as intermediate in MgO manufacture, it is first purified to obtain pure Mg(OH)2 prior to calcination to obtain MgO. Unfortunately, being slimy in nature, Mg(OH)2 as obtained in the precipitation reaction is difficult to filter and purification becomes more and more difficult to attain high levels of purity.